998 ON FOKM AND MECHANICAL EFFICIENCY [ch. 



In the case of our cantilever bridge, we shew the primitive girder 

 in Fig. 475, A, with its bending-moment diagram (B); and it is 

 evident that, if we turn this diagram upside down, it will still be 

 illustrative, just as before,, of the bending-moments from point 

 to point: for as yet it is merely a diagram, or graph, of relative 

 magnitudes. 



To either of these two stress diagrams, direct or inverted, we 

 may fit the design of the construction, as in Figs. 475, C and 476. 



Fig. 476. 



Now in different animals the amount and distribution of the 

 load differ so greatly that we can expect no single diagram, drawn 

 from the comparative anatomy of bridges, to apply equally well to 

 all the cases met with in the comparative anatomy of quadrupeds; 

 but nevertheless we have already gained an insight into the general 

 principles of "structural design" in the quadrupedal bridge. 



. In our last diagram the upper member of the cantilever is under 

 tension; it is represented in the quadruped by the ligamentum nuchae 

 on the one side of the cantilever, and by the supraspinous ligaments 

 of the dorsal vertebrae on the other. The compression-member 

 is similarly represented, on both sides of the cantilever, by the 

 vertebral column, or rather by the bodies of the vertebrae; while 

 the web, or "filling," of the girders, that is to say the upright or 

 sloping members which extend from one flange to the other, is 

 represented on the one hand by the spines of the vertebrae, and on 

 the other hand by the obli(|ue interspinous ligaments and muscles — 

 that is to say, by compression -members and tension-members 

 inclined in opposite directions to one another. The high spines over 

 the quadruped's withers are no other than the high sisruts which 

 rise over the supporting piers in the parabolic girder, and correspond 

 to the position of the maximal bending-moments. The fact that 

 these tall vertebrae of the withers usually slope backwards, some- 



